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History of the Atom Scientists and Their Contribution to

the Model of an Atom

Democritus (460 BC – 370 BC)

•  Proposed a theory of ATOMISM which states: –  Atoms are particles in space and make

up everything –  Atoms are in constant motion –  Atoms are indivisible –  Things differ in the universe because

of shape, arrangement and position of atoms

–  Every event results from a collision of atoms which make life predictable

•  Aristotle did not support his atomic theory

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chemistry.wikispaces.com/T.+Glenn+Time+Line+Project

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Aristotle (350 BC)

•  Rejected atomism •  Aristotle modified an earlier

theory that matter was made of four “elements”: earth, fire, water, air

•  Matter was continuous and not made of individual smaller particles

Aristotle

fire

air

water

earth

Antoine Lavoisier (1743 – 1794)

•  Known as the “Father of Modern Chemistry”

•  Discovered/proposed that combustion occurs when oxygen combines with other elements

•  Discovered/proposed the Law of Conservation of Mass (or Matter) which states, in a chemical reaction, matter is neither created nor destroyed

•  Thought matter is concrete and measureable

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geotime2.html

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John Dalton (1766 – 1844)

•  First Modern Atomic Theory called Chemical Atomic Theory or Billiard Ball Model a.  Elements of matter consist of characteristic

kinds of particles  b.  These particles of an element are identical to

each other  c.  Atoms are indivisible  d.  There are as many kinds of atoms as there

are elements  e.  Atoms of one element can’t be converted into

another  f.  Atoms in chemical reactions are neither

created or destroyed; merely rearranged  g.  Law of multiple proportions which describes

how atoms combine to make different compounds

•  Calculated the atomic weights of many various elements

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Dalton.htm

J.J. Thomson (1856 – 1940)

•  Proved that an atom can be divided into smaller parts

•  While experimenting with cathode-ray tubes, discovered corpuscles, which were later called electrons

•  Stated that the atom is neutral •  In 1897, proposed the Plum

Pudding Model which states that atoms mostly consist of positively charged material with negatively charged particles (electrons) located throughout the positive material

•  Won a Nobel Prize

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dayintech_0430

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Ernest Rutherford (1871 – 1937)

•  In 1909, performed the Gold Foil Experiment and suggested the following characteristics of the atom: o  Consists of a small core, or nucleus, that

contains most of the mass of the atom o  Nucleus is made up of particles called

protons, which have a positive charge o  Protons are surrounded by negatively

charged electrons, but most of the atom is actually empty space

•  Did extensive work on radioactivity (alpha & beta particles, gamma rays/waves) and was referred to as the “Father of Nuclear Physics”

•  His model is often called “Nuclear Model”

Image taken from: http://www.scientific-web.com/en/

Physics/Biographies/ErnestRutherford.html

Ernest Rutherford (movie1, movie2)

Most particles passed through. So, atoms are mostly empty.

Some positive α-particles deflected or bounced back!

Thus, a “nucleus” is positive & holds most of an atom’s mass.

Radioactive substance path of invisible

α-particles

•  Rutherford shot alpha (α) particles at gold foil.

Lead block Zinc sulfide screen Thin gold foil

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Niels Bohr (1885 – 1962)

•  In 1913, proposed the Bohr Model, which suggests that electrons travel around the nucleus of an atom in orbits or definite paths.

•  Additionally, the electrons can jump from a path in one level to a path in another level (depending on their energy)

•  Won a Nobel Prize •  Worked with Ernest Rutherford

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File:Niels_Bohr.jpg

Erwin Schrodinger (1887-1961)

•  In 1926, further explained the nature of electrons in an atom by stating exact location of an electron cannot be determined

•  It is more accurate to view the electrons in regions called electron clouds;

•  electron clouds are places where the electrons are likely to be found

•  Did extensive work on the Wave formula à Schrodinger equation

•  Won a Nobel Prize

Image taken from: nobelprize.org/.../1933/

schrodinger-bio.html

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James Chadwick (1891 – 1974)

•  Realized that the atomic mass of most elements was double the number of protons à discovery of the neutron in 1932

•  Worked on the Manhattan Project

•  Worked with Ernest Rutherford

•  Won a Nobel Prize

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dayintech_0227

Heisenberg

•  Came up with the Heisenberg Uncertainty Principle

•  It states that you can’t know BOTH the momentum and position of an electron at the same time

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Democritus proposes

the 1st atomic theory

460 – 370 BC

History of the Atom - Timeline Antoine Lavoisier

makes a substantial number of contributions

to the field of Chemistry

1766 – 1844

John Dalton proposes his

atomic theory in 1803 1743 – 1794

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1856 – 1940

J.J. Thomson discovers the electron and proposes the Plum Pudding Model in 1897 1871 – 1937

Ernest Rutherford performs the Gold Foil

Experiment in 1909

1885 – 1962

Niels Bohr proposes the Bohr Model in

1913 1887 – 1961

Erwin Schrodinger

describes the electron cloud in 1926

1891 – 1974

James Chadwick

discovered the neutron in in 1932

1700

s 18

00s

1900

s

Click on picture for more information

Progression of the Atomic Model

The structure of an atom, according to:

Democritus & John Dalton

J.J. Thomson Ernest Rutherford Neils Bohr Erwin Schrodinger James Chadwick

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